Sawing cords enjoy increased interest for sawing natural stone blocks into slabs for all kinds of applications. Sawing cords are replacing the traditional lamella and circular saw blades as they allow for higher linear speeds (typically 100 to 120 km/h) and hence higher cutting speed. Stone cutting machines with as much as 60 or more closed loops of sawing cords running parallel on grooved sheaves are being introduced nowadays. Such machines attain higher productivity and reduce the overall operational cost for the stone cutters compared to the existing gang saws and are replacing this technology at high pace.
In essence a sawing cord comprises three basic elements:                A central carrier cord that is made of steel filaments twisted together into a cord. The steel cord has a diameter of about 5 mm or 3.5 mm while lower diameters such as 3 mm or even 2 mm are being explored nowadays;        Sawing beads attached to the cord at regular distances. The number of beads per meter depends on the type of stone that needs to be cut. There are about 25 to 40 beads per meter on a sawing rope. The beads themselves exist out of a metal sleeve on which an abrasive layer is attached. This abrasive layer comprises diamond grit held in a metal matrix. The abrasive layer is currently obtained by powder metallurgical techniques although beads wherein the abrasive layer is applied by laser cladding are being tested. The total diameter of the bead is 7 or 11 mm depending on the application envisaged. The beads are threaded on the steel carrier rope in much the same way as pearls on a necklace;        The beads must be fixed to the carrier such that the motive force exerted on the steel rope is transferred to the bead. While in the past mechanical anchorage methods have been explored, the technology whereby beads are held in place by means of a polymer has prevailed. The polymer is injected in between the beads thereby forming sleeves surrounding the steel cord. In this way the steel cord is sealed from the coolant and the abrasive debris sawn away by the beads. A good adhesion chemistry helps to keep the sleeves fixed to the steel cord while holding the beads firmly in place. Bad polymer adhesion may lead to ‘bead collapse’: beads accumulate on the cord when one of them gets trapped in the cut.        
The three elements of a sawing cord must cooperate well together: the steel cord must have sufficient fatigue resistance, the sawing beads must gradually expose the diamond grit from the metal matrix while the polymer must keep its adhesion to the steel cord: a premature failure of any one of these results in a premature failure of the entire rope.
After analysing many failed sawing ropes, the inventors found that one of the predominant failure modes is cord failure at the end of the metal sleeve of the bead. One of the causes of this failure mode is that the rope is not exactly at the centre of the sleeve of the bead: the steel cord touches the metal sleeve. The effect of this contact is that the outer filaments of the steel cord will wear and corrode at that place leading to a premature failure of the rope. Furthermore an eccentric placement in the same radial direction of the steel rope in a series of beads may lead to a non-rotating sawing rope during use. For a uniform wear of the abrasive surface of the circular beads it is imperative that the beads rotate during use.
The reason why the cord is eccentrically placed is—according the inventors—that the polymer sleeves are coated on the steel rope by defective injection moulding. In injection moulding the beads threaded on the steel rope are positioned into a lower half-mould having an elongated recess corresponding to the negative of the sleeve one wants to obtain. At regular distances cavities for receiving the beads are provided. The upper half-mould (which is a mirror image of the lower half-mould) closes on the lower half-mould and polymer is injected into the recesses. After cooling the mould is opened, the finished sawing rope is taken out of the mould, shifted for positioning a new length of steel cord with beads and the injection cycle is repeated. An example of a mould is shown in FIG. 1 of US 2007/0194492 A1.
In U.S. Pat. No. 5,216,999 the eccentricity problem is recognised and solved by using an injection mould having annular protrusions (item 212 in FIG. 7) that keep the steel cord more in the centre during injection moulding. But still complete centring with this kind of mould is not possible as some clearance must remain between the cord and the annular protrusions as otherwise the cord would not be coated with polymer at the protrusions and would start to corrode there.
Eager to find a solution to this centricity problem, the inventors came up with the solution as described below.